The present invention relates to electron beam columns and more particularly to electron beam column apparatus and methods in which size and shape of a beam may be effectively varied during the operation of the electron beam column.
Electron beam columns have been adapted for use in systems for the microfabrication of large-scale integrated semiconductor circuits. For example, U.S. Pat. No. 3,644,700, issued Feb. 22, 1972, to Kruppa et al., describes an electron beam column adapted to form or "write" selected patterns on semiconductor wafers. Such columns have particular utility in the writing of such patterns on photoresists, i.e, exposing selected areas on photoresists which are then developed to form the photoresist masks extensively used in a wide variety of operations during integrated circuit fabrication. The typical electron beam columns utilized in connection with such integrated circuit microfabrication applications generally include an electron beam source, condenser lenses, alignment stages, demagnification lens stages, a projection lens, a deflection unit, and a target area, arranged in well-known fashion. Typical electron beam columns and components thereof are further described in U.S. Pat. No. 3,949,228, Ryan, issued Apr. 6, 1976, and U.S. Pat. No. 3,984,678, Loeffler et al, issued Oct. 5, 1976. Typical optical systems and components for such columns are further described in U.S. Pat. No. 3,930,181 and in the publication, "New Imaging and Deflection Concept for Probe-Forming Microfabrication Systems", H. C. Pfeiffer, J. Vac. Sci. Technol., November/December 1975, Vol. 12, No. 6, pp. 1170-1173.
The advantages of the square shaped electron beam over the more traditional Gaussian round beam has been set forth in detail in the above Pfeiffer article in the J. Vac. Sci. Technol. as well as in the above mentioned patents, U.S. Pat. Nos. 3,644,700 and 3,949,228. As set forth in these teachings, resolution and current density in electron optic systems are determined by the electron optical configuration and are effectively independent of the size of the target image. The Pfeiffer article indicates that a beam of relatively uniform intensity having twenty-five times the area of the usual Gaussian spot at approximately the same edge dose gradient can be obtained by projecting a square shaped beam onto a target.
This result will be summarized with respect to the comparison of the round beam and square shaped beam in FIG. 2 of the present drawings. The comparison illustrates the advantage of a square beam over a Gaussian round beam with identical resolution. In FIG. 2, the shape and size of a Gaussian beam spot 25 is compared to a square shaped beam spot 26 and in the graph beneath each beam spot layout the intensity distribution, i.e., intensity is plotted with respect to spot area. As set forth in the Pfeiffer article, in a conventional round-beam system, the intensity distribution halfwidth (d) equals the spatial resolution. The resolution of a Gaussian round beam is determined by the superposition of all n aberration disks .delta..sub.i plus the demagnified Gaussian source, which typically equals the quadrature sum of the aberrations for optimum current density: ##EQU1## To produce a true copy of the pattern, the half-width of the spot has to be at least five times smaller than the smallest element of the pattern. For the square spot, resolution is determined by the edge slope of the intensity distribution, caused only be the superposition of all n aberration disks: ##EQU2## The Gaussian disk of the source does not contribute to the edge shape. The size of the square spot is independent of resolution and can be chosen to match the smallest segment of the pattern. This entire segment is exposed at once, thereby speeding up the exposure rate by a factor of twentyfive over a comparable round-beam system.
It may be seen from FIG. 2 that using the square shaped beam, rectilinear area 27 (i.e., an area defined by straight lines) may be totally exposed by six stepped exposures (1 through 6) with square beam 26 while the same rectilinear area 27' would require in the order of one hundred and forty stepped exposures with the round Gaussian beam 25.
While present shaped or square beam systems provide significant electron beam capability in the integrated circuit fabrication field, it is foreseeable that in future technologies wherein portions of the patterns to be exposed may have dimensions below two microns, the application of the shaped electron beam in forming such patterns may be limited. In such dense integrated circuits having aperture widths and/or line widths with smallest dimensions below two microns, the "blooming effect" produced by double and greater multiple exposures to which some pattern areas may be subject when using shaped aperture apparatus may be beyond the dimensional tolerances of the integrated circuits. The problem of double exposure will be elaborated on in greater detail with respect to FIGS. 3 and 4. However, it may be seen in its simplest form with respect to area 27 which is exposed by the square shaped beam in FIG. 2. Utilizing a beam providing a square shaped spot 26, area 27 is exposed by six stepped exposures. Since the selected area to be exposed does not have dimensions which are integral multiples of spot 26, a portion 28 (crosshatched) will be double exposed. This double exposure results in the known "blooming effect" wherein the double exposed areas, for example, in the photoresist develop at a faster rate than the normal singly exposed areas during development. This produces undercutting and edge irregularity in the remaining photoresist defining the exposed area. With the present integrated circuits having line widths and apertures with smallest pattern dimensions of at least two microns, this "blooming effect" is within dimensional tolerances and presents no problems. However, with the denser, more advanced integrated circuits having lateral dimensions below two microns, the "blooming effect" may produce dimensional irregularities beyond the lateral tolerances.
In addition, in advanced integrated circuit technology, it would be highly desirable if the time required for electron beam exposure of selected patterns could be reduced thereby increasing the integrated circuit fabrication throughput.